Vibration isolating support



Janl, 1952 M. MCoNEY ETAL 2,580,815

' VIRATION ISOLATING SUPPORT Filed Dec. 28, 194s 2 SHEETS-SHEET 1 v ATTORNEY Jan. 1,-1952 M. MooNEY ETAL 2,580,815

VIBRATION ISOLATING SUPPORT Filed Dec. 28, 1942s4 2 SHEETS-SMT 2 ATTORNEY ting into suitable depressions in the plates I4 and I5. These universal joints permit the rods I3 to oscillate freely.

Our novel mechanism for isolating vertical vibrations is shown supported on the plate I5 in Figs. 1, 2 and 3, and on a larger scale in Fig. 4. The mechanism comprises two leaf springs 23, which carry the weightfof the instrument to be supported. The ends of these springs are clamped in slots 24 of adjustable retaining blocks 25 by means of screws 26. The blocks are adjustably attached to the plate I5 by means of screws 21 passing throughslots 28 (Fig.. 3). in

the plate. The distance between ends of fthev 4 placement is very small. In the event that the restoring force is very small and the supporting base of the isolating support vibrates horizontally, the horizontal vibratory force transmitted to the supporting plate I5 will also be very small.

The necessary relation between the height of the 'rodsf the length of the wires or other flexible suspension members, the weight supported by the rods, and the weight supported by the wires is given by the expression wherein H represents the height of the rods, L

-represents the ,length of the wires, R represents the weight supported by the rods, and W repre- The springs are fastened together at'their centers by nuts 32 on a threaded rod 33 which carries the load of the instrument pan II and the galvanometer I0.

The instrument pan II is attached to the rod 33 by a exible cord 34, which passes freely through a hole 35 in the plate I5. The thread on the upper end of the rod 33 makes it possible to adjust the vertical position of the rod so that the point of attachment 36 of the cord 34 thereto lies in the plane of the upper ends of the rods I3, which is the required condition for the most effective isolation of vibrations acting to oscillate or rotate the pan II and the instrument I0 carried thereby.

A suitable device 31 is associated with the galvanometer pan II and the plate I5 for damping horizontal and vertical movements of the pan II to cause it to come'to rest quickly if it 1s accidentally disturbed. Referring to Fig. 1 of the drawings, the damping device 31 comprises a pan 38 containing machine oil. A horizontal damping plate 39 is suspended in the oil from the plate I5 by means of the vertical rods 40 which pass freely through the holes 4I in the plate I5 (Fig. 4) and are attached to a crossbar 42. The cross-bar is resiliently supported on the plate I5 by a rubber cushion 43 which Vis supported on the plate.A The rubber cushion andthe cross-bar have holes 44 and 45 therein vwhich coincide with'hole 35 in the plate I5, and through which the rod 33 passes freely. A second pan of oil 46 rests on the 'horizontal damping plate 39. Vertical damping plates 41 extend from the bottom of the pan II into the oil'. The operation of this construction is such that if the supporting plate I5 or the pan II is accidentally disturbed the oil in pans 46 and 38 will act to restrain the movement of the damping plates 39 and 41, and 1 thus cause the system to come toV rest quickly. The rubber cushion 43 provides the preferred resilient connection between the damping plate 39 and the plate lI5 through the vertical rods 40 and the cross-bar 42. f

In describing the method of operation 0f our vibration isolating support, we will rst consider the method of isolating horizontalfvibrations. Referring to Fig 1 of the drawing, it is obvious that if the rods I3 alone supported the Weight of the plate I5,` it would topple over. On the other hand, if the wires I6 and leaf springs alone' supportedthe plate, it would hang suspended and would return to its position of rest if slightly displaced in any horizontal' direction.

With a proper combination of rod height, wirer sents the Weight'supported by the Wires. When thecharacteristics of the mechanism for isolating horizontal vibrations substantially satisfy this equation, it will be found that the natural horizontal period of the supported instrument I0 becomes infinite or extremely large, and horizontal vibrations, even those of very low frequency, are effectively isolated from the plate I5 and the instrument supported therefrom. In order to obtain conditions which substantially satisfy this equation, the correct length of the wires I8 is chosen to give approximately the correct ratio of load supported by the wires to that supported by the rods. The tension in the three wires can be equalized by adjusting screws I9. Final adjustment of the load ratio in' the rods I3 and wires I6 can be made by adding weights to the plate I5 untill the natural horizontal period of the system is one second or more.

Considering now the method of operation of our mechanism for isolating vertical vibrations, it is evident that the'leaf springs 23,' shown Vin Figs.`l, 2 and 4, havetheform of an inverted U when not loaded.` We have found that with a particular ratio of spring length to distance between the end clamping slots 24, the stressstrain vcurve (Fig. 6) lof this type of spring, when loaded' at its center', lis sigmoid (i. e., having one section of positive, the other of negative curvature form and horizontal at the point ofAinflection. Y The shape of the load-deflection curve is sh'ownin `Fig. 6, which shows the variation in deflection (expressed vas the ratio of the displacement 'of the spring to the length ofthe spring) withchanging load (expressed in grams). The necessary condition for complete isolation of the supported unit from vertical vibrations is simply that the`weight of the supported system must lequalthe force of the Vspring at the point of the" horizontal tangent of the load-v deflection curve, i. e., point X of Fig. 6.' When this condition obtains, the spring necessarily has an infinite naturalperiod of vertical oscillation, and therefore vertical vibrations, even of very low frequency',^are' substantiallyV eliminated by the spring, and the supported load is effectively protected Vfrom vertical vibrations of the base.

In'determining the dimensions of the spring to be used to support any given load, the equation *l I y is applied, where F is the load to be supported, S is the extended length of the spring, B is the exural rigidity of the spring, and K is a constant, determined by experiment to have a value 9i 50.4.11 the. Pericelarsmbodmeni 0f themvcntion illustrated# in. which? the endsf of 's the 'valueli'of K lf-will,"'vary with the. angle which. the 'spring Z3 'makes'with the` supporting: block'r25. The flexural. rigid-ity- B delinedlby theequation Y f .r2

where; -Y is. the lvoungismoumfus; bis the 'th-icas ness, and 'vvV isv 'the width of the spring. The distal-ice Vbetween f the end- -lamps'- y2l 'adjusted byV meansV 'Of-"ScreWSf-ZS, tofobtain the necessary rati'olof this distance to length-of spring' which wllfresult in 'a Ioaddeection rve having .the requ-ireusuapanviz., @curve-b incid'y forni which. is horizontal 'ati'theponi-ty 'of niiection as showin inm'g. 6.

Y IF-he` func "oni "of the cord-'3l is mainly' 'to-'preventv transmission of rotational vibr i'on's'tothe supported load.` This feature f 'our "structure, while highly desirable, is not vabsol'utly vessential in vevery case, since the rotatinaldisturbancesencountered in practice 'are fsmall.' "The cord 34 functions to prevent transmission lof rotational oscillatior-isY mosteif'ectively when the pointl 36 at which the -c'ord is attached lies in the plane of the top of the rods I3. Any rotatable iilament-like connection may be used instead of the cord, 34, e. g., a finefwire or a v'ry fine chain.

It will be understood by those skilled in the art that the particular embodiment of our invention which we have illustrated and .described in detail 'is' susceptible of 'rn-any' modifications. Thus, 4'the numberof rods l3fu'sed maybe 'modified, egg., four rods or more maybe Aused, although more than 3 rods are unnecessary. Likewise, more than three flexible suspension members f6 may be used if desired, and more than two overhead springs I8, or more than two leaf springs 23, may be used.

The flexible suspension members, instead of being wires, may be very ne chains, or rods Ahaving a universal joint at each end, or other suitable means.

In many applications, it may be desirable to use our novel spring for eliminating vertical vibrations in place of conventional leaf springs,

or clip springs, etc., for supporting apparatus or even very heavy machinery on which it is desired to absorb low frequency vibrations. In such cases one of our springs, or a plurality of our springs, may be used. In all cases the springs will be designed according to the principles set forth in detail above so that the ratio of the distance between the xed ends of the spring and the length of the spring is such that the loaddeflection curve has a point of inflection and a point of zero slope which coincide. The design load of the spring should be approximately at this point.

If desired, our device for eliminating horizontal vibrations, or our device for eliminating vertical vibrations, may be used in conjunction with known vibration isolating devices.

Having thus described our invention, what we claim and desire to protect by Letters Patent is:

1. A vibration isolating mechanism comprising a base, vertically extending rods supported on said base, a support for .an instrument to be isolated,

supported in part on the upper ends of said rods,

each end of said rods having a universal joint, flexible, resiliently supported suspension members attached to said support and adapted to carry the remaining Weight of said support and instrument, sand; an assembly "of" strip springs-femsnip k portingv the: instrument carried'rbyfsai'd; support, and having; ssentiallygan inverted U- shape .'When `unloaded, the dimensionali relations 'of I the parts. beingy such thatthe natural. frequency offvibra'- tio f? the :supportedsystem both? in horizontal an vertical *directions is-k substantially 2. Avibrationisolating mechanism for' isolating horizontal vibrations? comprising a'` base,` vertically 'extending rods V'universally .supported at one `end on 'said-f base, a support' for aninstrument to be isolated' universallysupported in par't'fon the/11p*- per endsof said rods,V flexible suspension mem' bersfattached to said support and adapted to carry theY 'remaining weight of` sai'd support' and instrument, andra resilientsupportforf-saidiiexiL lblesuspensionmembers.v

' 3f A spring adapted to isolatelow freduencywvlf. bratio'ns` comprising 4.a- -strip of, 'springina'terial x'ed ,ate'a'ch' end soasl to have essentiallyan 'in-j verted U shape when unloaded, the Ylength ofthe spring being given-bythe expression 1 B K c where A-F is the magnitude of La load to'bcsupf ported,l S" is the extended lengtnof the sfprin'g B i'sthe 'nexu'ral vrigidity oi the' spring, 'and' fa constant, said f lxed ends df they 'spring l being spaced; apart such .a distance tha-'tfwhen the spring is loaded at its center'with a load of F'magnitude; the'load-deection kcurvethereof is a sigmoid c'ive'fhavi'ng zero slope atthe point' of inflection.

'4.' A supporting mechanism 'for isolating' vibra:- tions comprising verticalrod's having a universal joint at each end carrying part of a 'weight'fto be supported, vertical flexible suspension members depending from an overhead resilient support carrying the balance of the weight to be supported, a horizontal load-supporting member carried by said vertical rods and said flexible suspension members, an assembly of strip springs arranged on said horizontal member, each of said strip springs being fixed at each end to said horizontal f member so as to have essentially an inverted U shape when unloaded, the relation between the distance between the xed ends of each strip spring and the length of each strip spring being such that the load-deflecton curve at the strip spring when loaded at its center is a sigmoid curve having zero slope at the point of inflection, the length, width, and thickness of the strip spring being such that the deflection of the strip spring when norm-a1 load is applied to the center of the strip spring substantially corresponds to the point of inflection of the load-deflection curve.

5. A supporting mechanism for isolating horizontal vibrations comprising vertical rods having a universal joint at each end and carrying part of the weight to be supported, and vertical nexible suspension members depending from an overhead resilient support carrying the balance of the Weight to be supported, the relation between the lengths of the rods and ilexible suspension members and the proportion of the weight carried respectively by the rods and by the suspension members being such that the mechanism has a long natural period of horizontal vibration.

6. A vibration isolating mechanism comprising a system for isolating horizontal vibrations including vertical rods having a universal joint at each end and carrying part of a weight to be supported, an overhead spring support, vertical flexible suspension members depending from said overhead spring support and carrying the balance assuma 7; of the .weight to be. supported, the relation between the lengths of the rods and flexible suspension members and the portion of the weight carried by the rods and suspension members being such that the mechanism has a long natural period in horizontal vibration, andra'mechanism for isolating vertical vibrations in combination with said horizontal vibration isolating mechanism'comprising a horizontal member carried by said vertical rods and said flexible suspension members. at least two strip springs xed at each end to the said horizontal member so as to have essentially an inverted U shape when unloaded, the relation between the length of each strip and the distance between the iixed ends of the strip being such that the load-deflection curve of the strip when loaded at its center is a sigmoid curve having zero slope at the point of inflection, the length, width, and thickness of the stripbeing such that the deflection of the spring when normal load is applied tothe center of the spring substantially corresponds to the point of inflection of the load-deilection curve, whereby the said vertical vibration isolating mechanism has a long natural period in vertical vibration.

7. A vibration isolating mechanism comprising a system for isolating horizontal vibrations including vertical rods having a universal joint at each end and carrying part of a weight to be supported, an overhead strip spring support, vertical flexible suspension members depending from said overhead strip spring support and carrying the balance of the weight to be supported,.the relation between the lengths of the rods and flexible suspension members and the proportion of the weight carried by the rods and suspension members being such that the mechanism has a long natural period in horizontal vibration, and a maV mechanism for isolating vertical vibrations4 in combination with the Vsaid horizontal vibration isolating mechanism jcomprlsing a horizontal member carried by said vertical rods and said exible suspension members, at least ,two strip springs xed at each end to the said horizontal member so as to have essentially an inverted U shape when unloaded, the relation between the length of each strip and the distance between the iixed ends of the strip being such that the load-deflection curve of the strip when loaded at its center is a sigmoid curve having zero slope at the point of inflection, the length, width. and thickness of the strip being such that the deection of the spring when normal load is applied to the center of the spring substantially corresponds to the point of inection of the load-deflection curve, whereby the said vertical vibration isolating mechanism -has a long natural period in vertical vibration, and a, rotatable filament-'like connection between the supported load and the aforesaid vertical vibration isolating mechanism, whereby rotational vibrations are substantially eliminated.

MELVIN MOONEY. ILEWELLYN E. COPELAND.

REFERENCES CITED The` following references are of record in the le of this patent: l

UNITED STATES PATENTS Number Name Date '720,042 Marshall Feb. 10, 1903 963,673 Weber July 5, 1910 1,162,138 Clark Nov. 30, 1915 1,811,250 Balduf June 23, 1931' 2,442,355 Greenslade June 1, 1948 

